xml feed vs xml document parsing -- the difference?

When parsing XML like below, does book.xml need to be a static file of xml or can I use a dynamic feed url of RSS? Does a feed URL need to end in XML or can it simply be the URL (like: http://feeds.voices.washingtonpost.com/wp/reliable-source/index)?

if (window.XMLHttpRequest)  {  xhttp=new XMLHttpRequest();  }else // IE 5/6  {  xhttp=new ActiveXObject("Microsoft.XMLHTTP");  }xhttp.open("GET","books.xml",false);xhttp.send();xmlDoc=xhttp.responseXML;